1. Field of the Invention
The present invention is directed to a respirator of the type wherein expiration pressure is identified by means of a pressure sensor, and a new inspiration phase is triggered when the expiration pressure falls below a prescribed value.
2. Description of the Prior Art
Respirators are known which include an inspiration line and an expiration line connectable to the respiratory paths of humans or animals, with the inspiration line having a first valve for setting the gas flow through the inspiration line, and the expiration line having a second valve for setting the gas flow through the expiration line. A control unit actuates the two valves in coordination so that a respiratory cycle having an inspiration phase and an expiration phase can be produced, the first valve remaining open to such an extent during the expiration phase that an adjustable bypass gas flow is produced. At least one pressure sensor identifies the expiration pressure and a trigger unit triggers a new inspiration phase during the course of the expiration phase when the pressure in the expiration line, as measured by the pressure sensor, falls below a prescribed value. A commercially available respirator is manufactured by Siemens known as Servo Ventilator 900 D (described in the Instruction Manual for the Servo Ventilator 900 D, Second German Edition, October 1988) which has an adjustable trigger level for the pressure in the expiration line. The trigger level is the relative underpressure which determines the pressure at which an inspiration shall be triggered. This pressure at which an inspiration is triggered is referred to as the trigger limit. The pressure in the expiration line is measured, and if the measured value falls below the trigger limit which has been set by virtue of, for example, a patient producing a corresponding underpressure due to an attempted inspiration, the respirator automatically switches from the expiration phase to the inspiration phase. In many instances, a positive end-expiration pressure (PEEP) is used during the expiration phase in order, for example, to prevent atelectasis. The PEEP range of adjustment, for example, may be from 0 through 50 mbar. The trigger level is thus dependent on the PEEP level. If, for example, a PEEP level of +10 mbar is selected and a trigger level of -2 mbar is selected, this means that a patient must produce an under pressure of 2 mbar, in comparison to the setting of the PEEP level, in order to trigger a breath. In other words, the trigger limit is +8 mbar.
The trigger level can be set to a fixed value by a rotary knob. Such setting can become difficult under certain circumstances. If the trigger level is set too low, i.e., if the spacing from the PEEP level to the trigger limit is selected too small, pressure fluctuations in the expiration line may cause (incorrectly) the triggering of a new breath, which should, of course, be avoided. Conversely, a trigger level which is too high, i.e., too large a spacing exists between the trigger limit and the PEEP level, can result in the patient having to exert a considerable force when attempting to breath, in order to generate the necessary underpressure in the expiration line. This is also undesirable. Due to the gas flow in the inspiration line which occurs even during the expiration phase, known as the bypass gas flow, the patient could initially inhale this gas quantity without the occurrence of a pressure drop in the expiration line.
The use of such a bypass gas flow is known, for example, from U.S. Pat. No. 4,401,116. The operation of a respirator with bypass gas flow is described in "Option 50" for the Model 7200a Ventilator manufactured by Puritan Bennett, Inc. This option is known as "flow-by". In contrast to the pressure-dependent triggering of a new breath described above, triggering dependent on gas flow is employed in this option. The gas flow in the inspiration line and in the expiration line is measured by means of two gas flow sensors, and a net gas flow is calculated therefrom. This corresponds to the patient gas flow, i.e., to the gas flow that is either absorbed when the patient inhales, or is expelled when the patient exhales. The net gas flow is zero during the respiratory pause, i.e., when the patient is neither inhaling nor exhaling.
When the patient begins to inhale, part of the bypass gas flow proceeds into the lungs of the patient, and the exhalation gas flow is correspondingly reduced. The net gas flow, or the patient gas flow, thus becomes negative. When this value reaches an adjustable sensitivity value, a new breath is triggered. For safety reasons, pressure-dependent triggering is always additionally provided as a back-up. These two trigger methods, which operate in parallel, make the setting and the manipulation of the ventilator even more complicated.